1. Field of the Invention
The present invention relates to a parallel transfer type director which enables transfer of a data item B between a disk cache unit and a main storage unit while a data item A is being transferred between a disk unit and the disk cache unit.
2. Description of the Prior Art
A disk cache unit has been developed to speed up the input/output processing of a computer system which will be limited by the I/O bound condition because the access speed of the central processing unit (CPU) of the computer is increased. The input/output processing speed is improved by utilizing a disk cache unit wherein the data that will be frequently referenced by the CPU is stored in the disk cache unit. Therefore, the locality of reference rarely takes place. (In general, the number of files being used among the files stored in a disk unit is quite small.) This means that the data residing in the vicinity of a data item that is referenced by the CPU is considered to belong to a file being currently used. Therefore, the data found in the proximity of the data referenced by the CPU is more frequently accessed as compared with the other data. In this aspect, the data existing near the data accessed by the CPU is fetched into the disk cache unit. Furthermore, if the access pattern is predetermined like that of the sequential access processing, the pertinent data can be read in advance.
Consequently, the processing explained above is imposed on a director of the disk controller unit when the disk cache unit is adopted; hence, the director becomes the new bottleneck of the computer system and the effect to be achieved by introducing the disk cache unit to the computer system cannot be satisfactorily obtained.